In an apparatus and method for hydroforming a dual wall conduit having a controlled size gap between the walls, a frame (11) has an upper crown (11C) and a lower bed (11D) defining a hydroforming space therebetween, the bed (11D) having a slideway extending from the space to a load-unload-preform position out of the space. A mold assembly on the slideway has a lower platen (17) and an upper platen (19) defining at least two hydroforming cavities (14,16), one cavity being an elongated preform and semi-finish cavity (14), and the other cavity being an elongated finish cavity (16). A mold shifting means (15) is positioned for shifting the mold assembly on the slideway from the position in the space between the crown (11C) and bed (11D), to and from the forward and load-unload-preform position. Mold closing and preforming hydraulic cylinders (27) are operably connected to the upper mold platen (19) for closing the upper platen (19) onto the lower platen (17) and creating mechanical preforming on dual wall tubular stock in the preform and semi-finish form cavity (14). The upper crown (11C) has a peripherally retained bladder (33) over the mold assembly for applying a closure clamping force on the mold assembly by the bladder (33). A first pair of double acting, tube sealing, hydroforming elements are at the ends of the preform, semi-finish cavity (14), and a second pair of tube sealing hydroforming elements (40) are at the ends of the finish cavity (16).